The proteins encoded in the most abundant mRNA class expressed in embryonic chick muscle have been identified. These include myosin, heavy chains, actins, tropomyosin, vimentin and the myosin light chains. Certain isozymic forms of these proteins are expressed only in differentiated muscle, e.g., alpha-actin and muscle specific myosin, where as other forms are expressed constitutively, e.g., beta, gamma-actin and fibroblast myosin. Using cloned dscDNA probes for the actin gene family we have isolated genomic sequences for the alpha, beta and gamma-actin genes from the Charon 4A library of the chicken genome. We are in the process of comparing the organization, structure and regulation of the differentiation specific vs the constitutive actin genes. dscDNA clones have been prepared for other isozymic muscle proteins and are being used to isolate the genomic sequences for similar studies.